[Plugin: WordPress Ultra Simple Paypal Shopping Cart] Shipping cost being added multiple times

I have a retail website which I’m trying to use this plugin with and customers are becoming confused when they add one item to their cart, then continue shopping and upon adding a second item to the cart, the shipping fee is being added again. And again with adding a 3rd item, and so on.

Is there a way to essentially “lock” the shipping calculation so that no additional shipping cost gets added when multiple items are added to the cart?

Here’s my site:
https://www.koshercakery.com

Your Thoughts?, Thanks!
Joe

https://www.ads-software.com/extend/plugins/wp-ultra-simple-paypal-shopping-cart/